The present invention relates to an apparatus for a continuous cooling or heating process for multistage changing of the ambient temperature, and, in particular, to an apparatus for a continuous freezing process.
The present invention also relates to a freezing method by use of the above apparatus which handles multistage changes in the ambient temperature, and to an apparatus for preparing a recording medium in which a program for controlling the operating conditions of method is written.
As methods for preserving foodstuffs, drying, salting, canning, retort pouching and the like are commonly known. However, in all of these methods the flavor and texture which typify the freshness are lost. On the other hand, freezing practically maintains the freshness of the food and is capable of preserving food for long periods, so can be considered as the ideal method of preservation.
Various methods of freezing preservation are known, such as air blast freezing, contact freezing, liquefied gas freezing, and the like.
In liquefied gas freezing, liquefied nitrogen or liquefied carbon dioxide, which evaporates in a short time under atmospheric pressure, is sprayed directly onto the material to be frozen, or the material is introduced into a refrigeration chamber into which the liquefied gas has been sprayed, and the material is frozen in an extremely short time. Because the freezing takes place in a very short time the quality of the resulting frozen food is good.
Liquified gas freezing is carried out by both batch and continuous processing methods. Continuous processing is superior from the aspect of productivity.
An apparatus used for continuous liquefied gas freezing is provided with a conveying member such as a net conveyor or the like which conveys the material to be frozen into a tunnel-shaped cooling chamber into which the liquefied gas is sprayed. The material is transported through the cooling chamber by the conveying member so that the material is subjected to a continuous freezing process.
However, the results obtained in this cooling process vary because of the size and heat conductivity of the material being frozen. Fish, meat agricultural products such as certain cereals including rice and the like, and food products such as cooked and frozen foodstuffs and the like vary in the frozen state because of their water content even though they are freezed under the same conditions. For that reason, with these foodstuffs, even when frozen in an atmosphere at the same temperature, the temperature of the food product does not drop uniformly. Especially between 0.degree. C. and -5.degree. C., the drop in the temperature of the product slows down. This region is called "maximum freezing-up temperature zone" in which the water throughout the product turns to ice so that ice crystals are formed. When freezing is retarded in this temperature region, the tissue in animal and plant products tends to break down, so that when the food product is thawed the juices in meat and the liquid in the plant cells are released and the natural fresh flavor is lost. For this reason rapid freezing is desirable. In addition to this, it is known that, in order to maintain the flavor and texture when preserving foods by freezing there are various ideal conditions to be met.
Accordingly, the freezing conditions are established in advance and the freezing process is carried out to conform to these conditions. For example, the ambient temperature during freezing should be changed in stages so that when the temperature of the food product goes from normal temperature to 0.degree. C. the ambient temperature is -30.degree. C. When the product temperature reaches the 0.degree. C. stage the ambient temperature should be changed to -100.degree. C., and when the -7.degree. C. level is reached in the product, -40.degree. C. is required.
In addition, a programmed freezing method is known, in which this type of freezing data is programmed in the recording medium in advance to control the liquefied gas freezing apparatus.
When a batch device is employed, it is easy to change the freezing conditions in a multistage way depending the kind of the foodstuff, but it is difficult to do so in a continuous unit. Specifically, in the case of a batch system, the freezing ambient temperature is changed in several stages to conform to the various foodstuffs, and the processing time is changed at each ambient temperature until the foodstuff reaches the desired temperature, so that freezing occurs easily.
However, in the case of a continuous process, the chamber in which the food product is transported is divided into a plurality of sections, but even though the ambient temperature of the freezing process can be changed in each section, the processing temperature is determined by the length of the partitioned refrigeration chamber, so that when a plurality of different products are to be processed in one apparatus, there is a limitation in that the optimum freezing conditions are set to conform to the individual products.
Also, in rapid freezing at ultralow temperatures the cooling program changes for each object material. For this reason, a great deal of specialized knowledge and labor is necessary in the preparation and the control of the program.
Centered around a freezing apparatus, the fact that multistage adjustment of the environment is necessary and the problem areas at that time, have been explained above. However, the circumstances are the same for other multistage temperature processes, such as, for example, with process apparatus for continuous, multistage environment adjustment at close to normal temperatures, used in the bio-industries for cultivating seedlings or strains of microorganisms, and the like; and with process apparatus for continuous multistage heating for drying or annealing and the like.